1. Field of the Invention
The present invention relates to a voice coil motor (VCM) apparatus for positioning, and in particular to an apparatus for positioning the movable lens of an auto-focus module used in a miniature camera.
2. The Related Art
A conventional voice coil motor includes a movable part and a stationary part as shown in FIG. 9. The stationary part, fixed on the substrate 92, is composed of a permanent magnet 911 and an iron yoke 912 to produce a magnetic field through the air gap 913 of the magnetic circuit. The movable part is composed of a coil winding 931 and a power cable 932 with one end being connected to the coil winding 931. When a current is supplied to the coil winding 931, the coil winding will move through the air gap as a result of the interaction between the current and the magnetic field.
As the coil winding 931 is the movable part of the voice coil motor, a flexible power cable 932, with one end connected to the coil winding 931 and the other connected to a power source on the substrate 92, should be used in order to draw in the necessary electricity during the motion of the motor. Conventional voice coil motors often employ a flexible U-shaped power cable. For a low-power voice coil motor, the power cable can be implemented with a flexible printed circuit board. Generally, the use of a U-shaped flexible printed circuit board requires a sufficiently large curvature radius, if the flexible printed circuit board is continuously twisted and able to last for a normal service life as expected. Most compact electronic devices simply do not have enough internal space to accommodate such requirements, and it is also against current efforts of downsizing components.
In addition, a positioning apparatus also needs a position detector 94 to measure the displacement of the movable part of the voice coil motor and a positioning controller 95 to adjust the coil current, so that the relative displacement between the movable part and the stationary part can be controlled.
Conventional voice coil motors have the following disadvantages when applied to the positioning of the lens for the auto-focus module of a miniature camera: (1) Conventional voice coil motors often employ non-contact position sensors such as linear scale, fiber-optic displacement detector, or laser displacement detector to feed back the displacement information of the movable part. However, for the application of a miniature camera, the sever limitation on the cost and the space will make the usage of the aforementioned position sensors almost impossible. (2) The flexible U-shaped power cable adopted in the voice coil motor needs sufficiently large space to maintain its life cycle under continuous twisting, which in turn will be against efforts to downsize components in a miniature camera. (3) Also, the additional force induced by the elasticity of the flexible power cable is another factor to be considered. The voice coil motor needs to draw more current to counter this force. This therefore results in considerable power consumption, which is a negative effect for the application of the voice coil motor in a battery-powered camera.
Therefore, developing a low-cost and small position feedback sensor and minimizing the negative effects due to the flexible power cable are some of the issues to be dealt with, if a voice coil motor is to be used in the positioning of the movable lens of miniature cameras.